The present invention relates generally to telephone networks, and more particularly to an interface between fiber optic transport media and a switching system designed for electrical signals.
As multimedia applications increase the demand for high-bandwidth, high-bit-rate communications, fiber optics technology is rapidly advancing to supply the capacity. A family of standards for optical fiber transmissions is known as the Synchronous Optical Network (SONET) standards. SONET was born as an extension to the DS1 hierarchy, which is a hierarchy of xe2x80x9celectricalxe2x80x9d as opposed to xe2x80x9copticalxe2x80x9d signals and consists of levels of signals formed by multiplexing lower level TDM (time division multiplex) signals.
The SONET standard establishes a multiplexing format for using any number of 51.84 Mbits/s signals as building blocks. An OC-3 (Optical Carrier, Level 3) is a 155.52 Mbits/s signal (3xc3x9751.84 Mbits/s), and its electrical signal counterpart is referred to as an STS-3 signal. The STS-1 signal carries a DS3 signal or a number of DS1 or other lower level signals. A SONET STS-3 signal is created by concatenating STS-1 signals. Each SONET STS-N electrical signal has a corresponding OC-N xe2x80x9coptical signalxe2x80x9d. The OC-N signals are created by converting the STS-N electrical signal to an optical signal.
Although optical switching techniques have been developed, telecom companies are eager to provide as much performance as possible from their existing infrastructure. Switching systems based on the DS1 electrical signal hierarchy are in place and continue to be used for signals carrying that type of signal. Essentially these switching systems use DS0 data, which is derived from the DS1 hierarchy. For example, a DS1 signal is comprised of 24 multiplexed DS0 voice channels. Thus, there is a demand for interfaces that will permit SONET signals to be switched through switching systems designed for the DS1 hierarchy of signals.
One aspect of the invention is a method of verifying connections for channels of network data within a delivery unit that provides an interface between telecommunications media and a switching matrix. A unique path verification code is assigned to each channel. The channels are formatted into subframes for transport within the delivery unit, each subframe containing one or more channels. Each channel in each subframe has an associated path verification (PV) bit. These PV bits are selected and sequenced, subframe by subframe, such that for each channel, a path verification code and a frame pattern are transported in a path verification superframe formed by accumulating path verification bits over a number of subframes.
At the end of a transport path, for each said channel, path verification monitoring is performed by detecting the frame pattern and comparing the received path verification code to an expected path verification code. A path verification error is registered if the comparison results in a mismatch.
In the example of this description, the path verification monitoring is performed by a state machine. This permits channels to be tracked regardless of their position in the subframe. Also, the use of a frame pattern permits monitoring to occur regardless of whether channels are in phase, as well as eliminates the need for global synchronization across channels.